This invention relates to a packaged reference liquid for calibration of blood gas-measuring equipment.
In the field of diagnostic medicine, the determinations of the partial pressures of carbon dioxide (PCO.sub.2) and oxygen (PO.sub.2) in the blood are clinically important. These measurements are an indication of respiratory efficiency, renal function, efficacy of inhalation therapy and the like.
The clinical laboratory has available to it a variety of instruments for conducting or performing these measurements. Typically these instruments require periodic calibration with standards or reference liquids.
The use of a reference standard for calibrating or checking the accuracy of a measuring device is wellknown. However, calibration of equipment designed to measure blood gases, such as carbon dioxide and oxygen, presents special problems. In the past, reference materials were not readily available and had to be prepared by the medical technologist immediately prior to performing the subject test. Generally, these materials were prepared by adding a gas mixture with known fractions of oxygen and carbon dioxide gases to a tonometer containing a control fluid sample at a buffered pH value. The gas and liquid phases were allowed to equilibrate within the tonometer, and an aliquot thereafter removed by the technician for use as a control for the blood gas instrumentation. Prior to the development of commercially available reference liquids, blood gas analyses could only be performed by trained technicians in specially equipped laboratories. Even when such reference liquids were available, they had a limited shelf life and would readily undergo changes in the dissolved gas concentrations upon exposure to the atmosphere.
Recently, packaged controls have become commercially available for blood gas testing. Such controls are provided in sealed containers, and do not have to be synthesized by a technician. Pre-packaged controls may be in the form of either single-phase (liquid) or two-phase (gas-liquid) products. A two-phase control generally requires equilibration at a constant temperature prior to use. Single phase reference liquids avoid the need for this equilibration, and thus are more convenient for the user.
U.S. Pat. No. 3,681,255 discloses the use of flexible, gas-tight containers containing a single-phase reference liquid. The liquid is prepared and packaged at ambient barometric pressures. These products suffer from a disadvantage, in that, under certain storage and transportation conditions, they may become unreliable. For example, air freight transportation conditions could easily involve situations where the external pressure would be so low that microbubbles would be formed in a liquid saturated with gases at standard atmospheric pressure.
In the system described in U.S. Pat. No. 4,116,336, there is an attempt to overcome the problem of microbubble formation. This system employs a subatmospheric pressure on the reference liquid at the time of preparation, filling and sealing. Thus, the total gas and vapor pressures in the liquid are below 600 mm Hg. Under most conditions, this system will not undergo phase separation. Preparation of these products is cumbersome, requiring extensive monitoring and pressure-controlling equipment and personnel. Additionally, microbubble formation, with the aforementioned untoward effects, could still occur should the packaged product be exposed to external pressures below 600 mm Hg.
U.S. Pat. No. 4,470,520 discloses a blood-gas quality control reagent, in a single liquid phase, stored in a gas impermeable tube with a valve attached for the release of the control into a blood-gas analyzer. This first tube is enclosed by a second gas-impermeable container, with the space between the two containers filled with compressed gas. This product is expensive and requires specialized manufacturing equipment. Additionally, it is not designed to be used with analyzers which have a sensor attached to an arterial line.
A need exists for a gas-equilibrated reference liquid that is convenient to prepare, affords the end user the assurance of a single-phase liquid reference regardless of transport, storage, or use conditions, and that may be safely used in an analyzer with a sensor attached to an arterial line.